Blue phases are virtually non-birefringent, but they exhibit the selective reflection of circularly polarised light, observed in chiral nematic phases. Whilst they are considered to be optically isotropic, they can become biaxial in the presence of an applied electric field, leading to field dependent optical density changes for incident light. These electro-optical effects can be viewed without the use of polarisers and with response times of 1 to 50 ms, depending on temperature.
U.S. Pat. No. 4,767,194 describes an optical system containing a blue phase liquid crystal layer in a cell to which an electric field can be applied transversely to the plane of the layer. A light source sends to the cell a beam of rays parallel to the field direction. The blue phase material is designed such that no selective reflection takes place for the light that is incident upon the device. The cell is optically isotropic in the presence of an electric field, switches rapidly and needs no interface orientation, and is suggested for use in intensity modulation of light of a given wavelength, or as a phase grid in a schlieren-optical system, an etalon in a laser, or a light switch. The blue phases of the liquid crystal materials of U.S. Pat. No. 4,767,194 are only described as exhibiting blue phases over narrow temperature ranges, for example BP I from 26 C. to 27 C., BP II from 27 C. to 28 C.
A paper by W. Cao et al in Nature Materials, Vol. 1, October 2002 at pages 111-113 describes mirrorless lasing in a 3-dimensional photonic band-gap, based on a blue phase liquid crystal material. This paper describes simultaneous lasing in 3-dimensions for a dye doped material in the BP II phase. The BP II liquid crystal phase is observed over the temperature range 23.6 C. to 24.4 C.
These devices based on blue phase liquid crystalline materials are of commercial interest, but commercial use has been restricted by the narrow operating temperature ranges of known blue phase liquid crystalline materials.
H. Kikuchi et al describe a method of broadening the temperature range of the blue phase BP I in Nature Materials, Vol. 1, September 2002 at pages 64-68. This paper describes the stabilisation of blue phases by polymer liquid crystal composites prepared by photopolymerisation of acrylate monomers in monomer/photoinitiator/liquid crystal mixtures in a blue phase state.
New formulations exhibiting broad temperature range blue phases, without needing to be stabilised as a composite, are the subject of the present invention.